1. Field of the Invention
The present invention relates to a combustion device, such as a water heater or a bath water heater, and in particular to a combustion device that while taking a temperature characteristic into account can detect a ventilation abnormality that is due to the blockage of a heat exchanger.
2. Related Arts
A combustion device, such as a water heater, supplies hot water on demand under the control of a controller that incorporates a sequential program for controlling a supply of hot water. In each instance, the controller calculates a calorific value to be supplied time after time in order to increase to a preset temperature the temperature of water that enters into a water heater. The supply of gas is adjusted in accordance with the combustion capability of gas having the demanded calorific value. In addition, the revolution rate of a combustion fan is controlled to supply an air-flow rate in accordance with the combustion capability. In this manner, an air-flow rate that is optimal for the combustion of gas is supplied to a burner, with the result that an air/gas ratio can be accurately controlled and complete combustion achieved.
The control of the air-flow rate is performed by using an air-flow rate sensor. The output data provided by the air-flow rate sensor are employed to acquire a difference between a required air-flow rate and an air-flow rate that is actually detected by the air-flow rate sensor. The revolution rate of the combustion fan is adjusted to correct for the acquired difference, and an optimal air-flow rate that matches the combustion value is supplied.
Commonly, when a water heater is used for an extended time period, fins in the heat exchanger of a water heater become blocked with dust, etc. As such blocking is a gradual and progressive process, resistance to the flow of ventilating air is gradually increased, accordingly. Finally, the air-flow rate required for the combustion of a burner can not be acquired even through the combustion fan is revolved at its maximum ability, and the end of the useful life of the water heater is reached. In other words, the end of the useful life has been reached means, in this case, that an abnormality has occurred in the combustion device and that the device should be either repaired, or discarded.
The present applicant proposed, in another patent application (International Patent Application No. PCT/JP95/01720 filed on Aug. 30, 1995), that the life of a water heater be determined based on a characteristic between the output of an air-flow rate sensor and the revolution rate of a combustion fan that is employed.
However, in case that the life of the combustion device is to be determined by self diagnosis when a burner is in the non-combustion state, the relationship between the air-flow rate, which is detected by the air-flow rate sensor, and the revolution rate of the combustion fan, which performs air-intake and flue ventilation, is affected by a change in the temperature of the ambient air or within the combustion device. That is, the revolution rate of the fan is linearly changed with respect to changes in the output data of the sensor; however, when the temperature (also called system temperature or ambient temperature) is changed while the data output by the sensor output are constant, the fan revolution rate is varied, even through the sensor output is constant. For example, when the ambient air temperature is raised from 0.degree. C. to 30.degree. C. while the data output by the sensor output are constant, the fan revolution rate is shifted (increased) by about 450 rpm, even though the sensor output is constant. This influence due to temperature is also apparent in the data output by the air-flow rate sensor when the fan revolution rate is constant.
As is described above, when the useful life of a water heater is determined during non-combustion, the characteristic relationship between the revolution rate of the combustion fan and the output by the air-flow rate sensor is varied due to change in the ambient air temperature or changes in the internal temperature of the device. Therefore, the useful life (or the existence of an abnormality) for the water heater can not be accurately determined.
Second, while the blocking of the heat exchanger by dust progresses gradually and occurs over a period of several months, the deterioration of the output characteristic of the sensor, like the air-flow rate sensor, etc., is commonly progressive over a period of a year. The final deterioration of the sensor output characteristic, however, may occur momentarily during an earthquake or upon the impact of an object, and thus the momentary deterioration of the sensor output characteristic must be detected in advance. This must be done because adequate combustion control and the above described determination of the useful life or of the existence of an abnormality can be performed by using correct sensor output data.
The deterioration of the output data characteristic of the air-flow rate sensor is also detected by first setting the revolution rate of the combustion fan to a predetermined value, and then checking the sensor output data at that time; or by first controlling the fan revolution rate in order to set the output data of the air-flow rate sensor to a predetermined value, and then checking the fan revolution rate at that time. In this case, also, there is the influence due to temperature changes.
Third, the influence due to temperature changes must be considered in order to detect a dust blockage or a deterioration of the sensor output data characteristic by referring to the relationship of the revolution rate of the combustion fan and the air flow rate. It is therefore desirable that the state of the ventilation be examined when there is no combustion. However, as there is a variation in the temperature in the interior of the combustion device right after the combustion is terminated, an adequate examination is difficult.
Fourth, during the examination of the characteristic in the non-combustion state, a user may sometimes use the combustion device, and the characteristic examination may inconvenience the user.